System and method for using a flexible composite surface for pressure-drop free heat transfer enhancement and flow drag reduction

ABSTRACT

A flexible composite sheet is disclosed. The flexible composite sheet (FCS) comprising a membrane, a substrate coupled to the membrane, and a plurality of ridges coupled between the membrane and the substrate, wherein a vibratory motion is induced from the flow to at least one segment of a membrane spanning a distance s, wherein the vibratory motion is reflected from at least one segment of the membrane to the flow, and wherein a reduction in fluctuations is caused in the flow pressure gradient and freestream velocity U at all frequencies except around f, where f≈U/s. When coupled to a heat exchanger fin, the FCS can enhance heat transfer without enhancing flow pressure drop. The FCS has other flow control applications, such as drag reduction when coupled to an aircraft wing.

FIELD OF THE INVENTION

[0001] The present invention relates to a passive flow-control methodfor customizing turbulent flow fluctuations, and more particularly to aflexible composite surface for achieving this and resulting in enhancingheat transfer in heat exchanger passages while minimizing the drop inflow pressure, and reducing fluid flow induced drag.

BACKGROUND OF THE INVENTION

[0002] Heat exchangers are used for transferring heat in a variety ofsystems such as those for manufacturing, heating ventilating andair-conditioning, power generation, and electronic packaging. One goalin the design of a heat exchanger is to maximize the convective heattransfer between a working fluid and a solid wall. One way to do this isby increasing the velocity of the fluid, which enhances the wallconvective heat transfer coefficient. However, as per the estimates ofKays and London (1984), while the heat transfer coefficient is directlyproportional to the velocity, the power required to drive the flow isproportional to the square of the velocity. This imposes an upper limiton the maximum allowable velocities in the heat exchanger.

[0003] Most compact heat exchangers employ closely spaced fins orsimilar structures to augment the heat transfer area for a given devicevolume. Additional augmentation requires modifying the wall boundarylayer flow, usually with the help of turbulence promoters, such asbaffles or wall roughness elements. This is generally necessary for heatexchange from air streams due to significantly lower heat capacities andthermal conductivities of air compared to water or other commonly usedliquid heat transfer media.

[0004] The principal problem of this solution is that using suchturbulence promoters causes a significant drop in flow pressure, therebyincreasing the power consumption of the fans. A second drawback is thatturbulence promoters often snag solid particles or debris, therebyincreasing flow blockage and heat transfer surface fouling in manyinstances.

[0005] Generally, there is not a good solution to these problems.Accordingly, what is needed is a system and method for increasing heattransfer while minimizing, or eliminating the additional flow pressuredrop. The present invention addresses such a need.

SUMMARY OF THE INVENTION

[0006] A flexible composite sheet is disclosed. The flexible compositesheet comprising a membrane, a substrate coupled to the membrane, and aplurality of ridges coupled between the membrane and the substrate,wherein a vibratory motion is induced from the flow to at least onesegment of a membrane spanning a distances, wherein the vibratory motionis reflected from at least one segment of the membrane to the flow, and;wherein a reduction in fluctuations is caused in the flow pressuregradient and freestream velocity U at all frequencies except around f,where f≈U/s.

[0007] In one embodiment, the flexible composite sheet can be wrappedaround a blunt leading edge of a plate facing an incoming flow of fluid.In another embodiment, the flexible composite sheet can also be wrappedaround one or more regions of an aerodynamic surface where a flowpressure gradient changes from favorable to adverse. In anotherembodiment, the flexible composite sheet is replaced with a plurality ofplates coupled to a substrate, wherein the plurality of plates has edgesthat interact with a fluid flow similar to a compliant surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a diagram of a flexible composite surface (FCS) inaccordance with the present invention;

[0009]FIG. 2 is a diagram of a portion of the FCS of FIG. 1 interactingwith a flow of fluid in accordance with the present invention;

[0010]FIG. 3 shows a photograph of a Global-GT3 test aircraft;

[0011]FIG. 4 is a diagram showing the cross-section of the wing of FIG.3;

[0012]FIG. 5 shows a photograph of an FCS mounted on the bottom of thewing of FIG. 3;

[0013]FIG. 6 is a chart showing measured pressure-side boundary-layervelocity profiles at 80% of chord from the leading edge, with andwithout the FCS;

[0014]FIG. 7 is a chart showing measured suction-side boundary-layervelocity profiles, with and without the FCS at 80% of chord from theleading edge;

[0015]FIG. 8 is a chart showing plots of the pressure-side velocity dataof FIG. 6 normalized with respect to the measured velocities furthestaway from the wall;

[0016]FIG. 9 is a diagram of an FCS interacting with a flow of fluid inaccordance with another embodiment of the present invention;

[0017]FIG. 10 is a blow-up diagram of a portion of the FCS of FIG. 9interacting with a flow of fluid in accordance with another embodimentof the present invention;

[0018]FIG. 11 is a diagram an FCS interacting with a flow of fluid inaccordance with another embodiment of the present invention;

[0019]FIG. 12 is a diagram of a heat transfer enhancement test apparatusin accordance with another embodiment of the present invention;

[0020]FIG. 13 is a top-view diagram of a multi-fin heat sink inaccordance with another embodiment of the present invention; and

[0021]FIG. 14 is a side-view diagram of the multi-fin heat sink of FIG.13.

DETAILED DESCRIPTION

[0022] The present invention relates to heat exchangers, and moreparticularly to a flexible composite surface for enhancing heat transferin heat exchanger passages while minimizing the drop in flow pressure.The following description is presented to enable one of ordinary skillin the art to make and use the invention and is provided in the contextof a patent application and its requirements. Various modifications tothe preferred embodiment and the generic principles and featuresdescribed herein will be readily apparent to those skilled in the art.Thus, the present invention is not intended to be limited to theembodiment shown but is to be accorded the widest scope consistent withthe principles and features described herein.

[0023] Generally, a system and method in accordance with the presentinvention enhances the transfer of heat in heat exchangers by utilizinga flexible composite surface (FCS). The FCS includes a membrane coupledto a substrate and a plurality of ridges coupled between the membraneand the substrate. Vibratory motion from a flow pressure gradientfluctuation is applied to at least one segment of the membrane. Themembrane reflects the vibratory motion from the at least one of itssegments to the flow pressure gradient fluctuation. This sustainsfluctuations in the flow pressure gradient only around a pre-selectedfrequency. This helps sustain a thin layer of re-circulating fluiddownstream of the FCS over the solid surface, which exchanges heat withthe flow. This thin layer allows efficient heat transfer from the solidsurface to the flowing fluid without introducing high frictional forcesbetween the fluid and the wall. This allows heat transfer withoutincreasing the pressure drop in the fluid flow passage. To moreparticularly describe the features of the present invention, refer nowto the following description in conjunction with the accompanyingfigures.

[0024]FIG. 1 is a diagram of a flexible composite surface (FCS) 100 inaccordance with the present invention. The FCS 100 is also referred toas the SINHA-FCS 100. The FCS includes a flexible membrane 102, which isstretched across an array of strips or ridges 104. The ridges 104 arecoupled to a substrate 106. The FCS 100 can be coupled to an aerodynamicbody. In this specific embodiment, the FCS 100 is coupled to a surfaceof a wing 108. Also, the membrane 102 is thinner (e.g., 6 um) than thesubstrate base (e.g., 50-100 um).

[0025] The membrane 102, the ridges 104, and the substrate 106 form airpockets 110 that contribute towards the stiffness and damping governingflexural vibratory motion 112 of the membrane 102. The flexuralvibratory motion 112 is caused by the flow 114 of a fluid along themembrane 102.

[0026] The natural frequency of the flexural vibratory motion 112 can betuned as desired by varying the spacing S between the ridges 104, thesize (e.g., thickness) of the air pockets, the tension of the membrane102, as well as the density and elastic modulus of the membrane material(Sinha et al, 1999). The damping of the membrane 102 can be made to varywith frequency and flexural mode by segmenting the air pockets 110 withsuitably located shorter ridges. The narrow gap above a short ridgeprovides an increased resistance to airflow across it. Thus, allflexural modes of the membrane requiring such flows in the substratehave larger damping in comparison to modes that do not. One benefit ofthe FCS 100 is that it controls the frequency and flexural modepassively, i.e., non-powered.

[0027] As will is illustrated in more detail below, the mechanics of theinteraction between the FCS 100 and the flow 114 stems from the flow 114imparting motion to the membrane 102 and vice versa. Even though thefull details of such interaction are extremely complex, certain dominantinteraction modes can be extracted by properly tailoring the mechanicalproperties of the membrane 102 in relationship to key features of theflow 114, such as the pressure gradient.

[0028] The FCS 100 exploits such a dominant interaction mode formanipulating a varying and adverse-pressure gradient (APG) boundarylayer flow. APG flows are those where the imposed pressure tends tooppose the flow. In many instances, this leads to boundary layer flowseparation, resulting in large increases in turbulence and flow losses.The present invention decreases the boundary layer flow separation andthus decreases overall turbulence and flow losses. As a result of suchmanipulation, any turbulence in the flow 114 is controlled and thetransfer of momentum, heat, and mass across the APG boundary layer canbe decoupled and changed to obtain desired outcomes.

[0029] Almost all turbulent frequencies can be controlled or eliminated.Also, a small selected frequency band can be amplified, therebycustomizing the spectrum of the turbulent fluctuations. Such a selectivemodification of the turbulent spectrum is another benefit of theembodiments of the present invention. Another benefit is that the FCScan interact with an inflectional velocity profile downstream of thepoint of flexible-wall interaction.

[0030]FIG. 2 is a diagram of a portion of the FCS 100 of FIG. 1interacting with a flow 114 of fluid in accordance with the presentinvention. The FCS 100 can be located over regions of an aerodynamicsurface where the flow pressure gradient changes from favorable toadverse. Under such flow conditions, flow induced pressure fluctuationscan impart flexural vibratory motion 112 to segments of the membrane 102between adjacent ridges 104. The flexural vibratory motion 112 of themembrane segments, in turn, can impart pressure fluctuations to the flow114 at the vibrating frequencies. This interaction constrains thepressure fluctuations and the resulting flow velocity fluctuationsaround a frequency f≈U/s (where, U=the freestream velocity above themembrane and s=the distance between adjacent high ridges on thesubstrate), as long as f does not coincide with the fundamental flexuralnatural frequency of the vibrating membrane segment.

[0031] The exposed surface of the membrane 102 creates a non-zero wallvelocity condition for the boundary layer flow at locations where theflow 114 is receptive to this condition. The interaction of the flow 114with the flexural vibratory motion 112 of the compliant membrane 102results in the flow 114 being forced to a new equilibrium.

[0032] The following description elucidates details crucial towardsexploiting this interaction. The streamwise u-momentum equation of theflow 114 at the mean equilibrium position (y=0) of the surface of themembrane 102 of the FCS 100 is considered first:

v(∂u/∂y)y=0=−(1/ρ)(∂p/∂x)+(μ/ρ)(∂2u/∂y2)y=0   (1)

[0033] The streamwise x-component of velocity “u” of the vibratingmembrane 102 (or the velocity of the fluid at the points of contact withthe membrane 102) has been assumed to be negligible, while thewall-normal y-component of velocity “v” of the fluid next to themembrane 102 is clearly non-zero due to membrane compliance. Key toflow-membrane interaction is the realization that the wall-normalgradient of the streamwise velocity at the wall, (∂u/∂y)y=0, can beextremely large at certain x-locations. At such locations, even a smalloscillation velocity (v<<U) of the flexible membrane can make thev(∂u/∂y)y=0 “control” term on the left hand side of equation (1)predominant. For a non-porous, non-compliant wall, this control term isidentically zero. Additionally, if the boundary layer velocity profileat the aforementioned locations is such that prior to interaction(∂²u/∂y²) y=0≈0, while |(∂u/∂y)y=0|>0, (i.e., u(y) is approximatelylinear near the wall) an order of magnitude balance of the terms inequation (1) yields:

v(∂u/∂y)y=0≈−(1/ρ)(∂p/∂x)  (1-a)

[0034] Such a condition can be satisfied in boundary layers over curvedsurfaces, in the vicinity of x-locations where the streamwise pressuregradient ∂p/∂x changes from favorable (∂p/∂x<0) to adverse (∂p/∂x>0), asshown in FIGS. 1 and 2. What makes such locations unique is the largerelative change in ∂p/∂x introduced through equation (1-a), since∂p/∂x≈0 prior to this interaction.

[0035] For boundary layer flows, pressure variation across the boundarylayer (∂p/∂y) is negligible, and the streamwise pressure gradient ∂p/∂xcan be obtained from the inviscid momentum equation at the outer, orfreestream edge of the boundary layer:

(∂U/∂t)+U(∂U/∂x)=−(1/ρ)(∂p/∂x)  (2)

[0036] For x-locations where equation (1-a) holds, an oscillatory motionof the wall can, therefore, directly introduce fluctuations in thefreestream velocity U, through the pressure gradient term. For example,in a steady boundary layer flow over a rigid non-porous wall, thepressure gradient term on the right hand side of equation (2) will becompletely balanced by the non-linear convective term [U (∂U/∂x)] on theleft hand side. If this flow is perturbed, by introducing a smallwall-normal velocity v through flexible wall motion, the resultingfluctuations in the pressure gradient will have to be balanced by theunsteady term (∂U/∂t) in equation (2). For x-locations where ∂p/∂x≈0 inthe unmodified flow, as required for ensuring the validity of equation(1-a), the overall effect of wall motion can be expressed as:

∂U/∂t≈v(∂u/∂y)y=0  (3)

[0037] It is important to note that equation (3) holds irrespective ofthe source of the perturbations. The discussions thus far have presumedthe source to the flexible wall (Sinha, 2001). However, equation (3)also describes how fluctuations in the freestream velocity U can impartoscillations to a compliant wall at x-locations where equation (1-a)remains valid (Sinha and Zou, 2000). If fluctuations exist in thefreestream velocity U, as is normally the case in most externalaerodynamic flows, the presence of a compliant wall around the ∂p/∂x ≈0location results in partitioning the energy of the fluctuations betweenthe fluid and the wall (Carpenter et al, 2001). The degree ofpartitioning at any instant depends on the temporal phase of the walloscillation cycle.

[0038] The vibratory response of the wall also plays a key role in thisinteraction. The predominant response of the FCS 100 can be expected tobe flexural. The maximum displacements and energy storage capacity ofthe FCS 100 corresponds to the fundamental mode as per the sketch of thedeflected membrane in FIGS. 1 and 2. Dissipation can also be expected tobe higher for higher modes of flexural vibratory motion, especially ifthe low ridges constrict the airflow across them.

[0039] The combined flow-wall interaction proceeds as follows: As a massof disturbed freestream fluid approaches a segment of the membrane 102,where equation (1-a) holds, the membrane 102 begins to undergo flexuraldisplacement. The membrane 102 continues to deflect as the disturbedfluid convects over it. At some point the displaced membrane 102 beginsto swing back, initiating the reverse phase of the oscillation cycle. Inthe process of deflecting to its extreme position, the membrane 102 andsubstrate 106 of the FCS 100 store a significant portion of the flowfluctuation kinetic energy as elastic potential energy. As the membrane102 springs back, most of this energy is released back to the flow 114.However, the original fluid particles, which had provided this energy,would have convected downstream by a distance U.Δt during the timeinterval Δt taken by the membrane 102 to execute one oscillation cycle.For the re-released energy to be imparted to the same mass of fluid thatoriginated it, the following condition must hold:

U.Δt=s  (4)

[0040] where, s=the free length of the membrane of the FCS 100, betweentwo ridges. This condition imposes the membrane oscillation frequency:f=U/s. The aforementioned process results in amplifying fluctuationscorresponding to f, while attenuating fluctuations at other frequencies.The efficacy of the selection process depends on the ability of the FCSto damp out higher modes, while minimizing damping in the fundamentalflexural mode. Also, the spacing s has to be sufficiently close suchthat equations (1-a) and (3) hold throughout this region. The frequencyselection criterion and the conditions needed for small amplitude wallmotion to influence the freestream also hold for externally actuatedactive flexible wall transducers (Sinha, 1999 and Sinha, 2001). Thevalidity of equation (3) has been experimentally verified by noting thefact that electrically driven flexible wall motion at a frequency f=U/sproduced large fluctuations in the freestream velocity U at the samefrequency while attenuating fluctuations at other frequencies (Sinha,2001).

[0041] The net effect of the aforementioned selection process is toconcentrate velocity and pressure fluctuations at a frequency f≈U/s.Also, these fluctuations convect downstream to the point where theboundary layer begins to separate. At the separation point, equation (1)simplifies to:

0=−(1/ρ)(∂p/∂x)+(μ/ρ)(∂ 2u/∂y 2)y=0  (5)

[0042] This implies that fluctuations in ∂p/∂x directly contributetowards introducing a vorticity flux ∂Ω/∂y+∂(∂u/∂y)/∂y through theviscous term in equation (5). Also, equations (1-a) and (3) hold on thecenterline of the separated shear layer, immediately downstream of theseparation point. The final effect is to utilize sustained fluctuationsin the freestream velocity U to impart wall-normal oscillations at apredetermined frequency U/s to the separated shear layer, therebyencouraging rapid entrainment of the surrounding fluid through wavebreaking. Increased entrainment from the separated region near the wallreduces the pressure in this region and forces the separated shear layercloser to the wall. This results in reattachment of the flow.

[0043] Compared to the unmodified flow, the FCS 100 constrains turbulentfluctuations to a narrower band. This “customized turbulence” can beexpected to be less dissipative. The fundamental natural frequency forflexural vibratory motions 112 of the membrane 102 has no bearing on theflow-membrane interaction frequency f, as long as they are sufficientlyapart. If the two coincide, the amplitude of the oscillating membrane102 increases, thereby enhancing non-linear dynamic effects. This cantrigger other modes of oscillation of the membrane 102, therebyincreasing energy losses and broadening the spectrum of flowfluctuations. The FCS 100 then begins to behave as a broad-spectrumturbulator, promoting much larger losses through rapid buildup ofturbulent skin friction.

[0044] One of the features of the FCS 100 is control of boundary layerflows in general, including applications to aircraft wings. The FCS 100can be applied to an aircraft wing to achieve drag reduction. In orderto ascertain the feasibility of using the FCS 100 to reduce wing dragflight tests were conducted with an FCS tape (with 0.4 mm-wide highstrips with spacing s=0.8 mm and a single 15-μm lower low strip in thecenter of each pair of high strips) mounted at about 65-75% of a chordfrom a leading edge on the top (suction) and bottom (pressure) surfacesof an advanced 1.24-m chord.

[0045]FIG. 3 shows a photograph of a Global-GT3 test aircraft 140(manufactured by Global Aircraft Inc., Starkville, Miss.), which isinstrumented for wing-bottom measurements. The aircraft 140 has a wing150, which is used for the wing drag flight tests. The wing 150 has astarboard flap 152. Pressure transducer array 154 is mounted on top ofthe wing 150.

[0046]FIG. 4 is a diagram showing the cross-section of the wing 150 ofFIG. 3. In this specific embodiment, the wing 150 is an NLF-0414Fnatural laminar-flow airfoil wing. The flow pressure gradient changesfrom slightly favorable to adverse around 65-75% of the chord on boththe top (suction) and bottom (pressure) surfaces of this airfoil.

[0047]FIG. 5 shows a photograph of a SINHA-FCS 100 mounted on the bottomof the wing 150 of FIG. 3, along with the boundary layer mouse 160 usedto measure boundary layer velocity profiles. This arrangement is justbelow the outboard end of the taped section of the starboard flap 152 ofFIG. 3. In this specific application, the FCS 100 is a 300-mm spanwiseand 50-mm chordwise section. A wing-flap joint 162 runs over the mouse160. The leading edge of mouse tubes 166 are immediately upstream of thewing-flap joint 164 at x/c=0.8.

[0048] During the test, the aircraft was flown at about 3000 ft pressurealtitude at its level cruising speed of 106-kt. This corresponded toRec≅4.8×106, flight Mach number M≅0.22 and a section angle of attackα≅−1°. Several sets of data were acquired both for the clean airplanewithout the FCS 100, as well as with the FCS 100.

[0049]FIG. 6 is a chart showing measured pressure-side boundary-layervelocity profiles at 80% of chord from the leading edge, with andwithout the SINHA-FCS. Integrating the velocity profiles shows the dragresulting from the marginal separation induced wake momentum defect as:Fractional reduction in drag=[∫ρu2dy|with FCS−∫ρu2dy |cleanwing]/∫ρu2dy|clean wing. The data in FIG. 6 showed that the FCS reducedthe drag under level cruise conditions by about 25%.

[0050] A significant increase in the freestream velocity is also seendue to the FCS. This could not be attributed to measurementuncertainties. The FCS, therefore, also helps speed up the flow outsidethe viscous dominated boundary layer. As expected for a lifting wing, atx/c=0.8, the freestream velocities on the suction side are higher thanthose on the pressure side. However the difference is smaller for thedata with the FCS. Hence, it is possible for the FCS to influence CL aswell.

[0051]FIG. 7 is a chart showing measured suction-side boundary-layervelocity profiles, with and without the SINHA-FCS at 80% of chord fromthe leading edge. The difference between Clean-Wing-1 and Clean-Wing-2profiles shows test uncertainties. FIG. 7 shows a similar behavior forthe suction side of the wing, resulting in 18-20% reduction in drag. Thetwo “Clean-Wing” profiles, corresponding to the extreme values of themeasured velocity profiles, provide a visual indication of uncertaintiesin the acquired data due to unavoidable atmospheric turbulence. Based onthe aforementioned estimates from this data, approximately 20% reductionin wing drag can be expected for the section of the wing influenced bythe FCS if it is affixed to both top and bottom surfaces.

[0052] The data of FIGS. 6 and 7 were obtained by affixing the FCS stripfirst to the pressure side only and then to the suction side only. Ifthe FCS were applied to cover substantial spanwise locations on bothsurfaces, the wing angle of attack and the throttle setting wouldprobably have to be changed to maintain the constant 106-kt airspeed.

[0053]FIG. 8 is a chart showing plots of the pressure-side velocity dataof FIG. 6 normalized with respect to the measured velocities furthestaway from the wall. The profiles for the wing with FCS are normalizedwith respect to δ* values before and after FCS application. Thisisolates the change in the shape of the velocity profile. FIG. 8demonstrates that applying the FCS on the bottom surface reduces theshape factor H (H=displacement thickness δ*/momentum thicknessθ) from1.46 to 1.35, thereby making it fuller.

[0054]FIG. 9 is a diagram of an FCS 200 interacting with a flow of fluidin accordance with an embodiment of the present invention. Thisembodiment consists of thin plates 202 staggered at a shallow angle andsandwiched between compliant porous elastomeric layers 204 havingvisco-elastic properties. This assembly is imbedded in a substrate 206,which can be affixed to a body over which an adverse-pressure-gradientflow 208 takes place.

[0055]FIG. 10 is a blow-up diagram of a portion of the FCS 200 of FIG. 9interacting with a flow of fluid in accordance with another embodimentof the present invention. The tips 220 of the plates 202 are exposed toa locally varying pressure gradient, changing from favorable upstream toadverse downstream. In a manner similar to the previous embodiment, thetips 220 will experience flow-induced oscillations, since the flowpressure gradient exactly over it will be zero. The flow 208 willseparate downstream of the tips 220 entrapping a small vortex 222. Dueto the damping provided by the compliant layers 204, most of theturbulent kinetic energy imparted to the plates 202 will be dissipated.However, in a manner similar to the previous embodiment, flow-inducedoscillations around the frequency f≈U/s (U=the local freestream velocityof the flow 208, and s=streamwise spacing of the plate tips 220) will beallowed to pass. This will control the entrainment in a shear layer 224.

[0056] In the ideal case, the vortex 222 should extend just up to thetip 220 of the plate 202 immediately downstream. A larger vortex 222will cause full-blown flow separation with an accompanying largeincrease in form or pressure drag. Whereas, a small vortex 222, due toexcessive entrainment in the shear layer 224, will increase the skinfriction drag. A reduction in skin friction occurs due to the reversedflow next to the surface of the plates 202 caused by the vortex 222. Thechoice of the compliant porous elastomeric layer has to be such that itsdamping increases significantly for oscillation frequencies greater than2f.

[0057]FIG. 11 is a diagram of an FCS 250 interacting with a flow offluid in accordance with another embodiment of the present invention. Inthis specific embodiment, the plates 202 have a curved profile givingand form a fish-scale pattern. As such, counter-rotating longitudinalvortices 252 can be generated that can assist in drawing the shear layercloser to the surface of the plates 202 by enhancing mixing.

[0058]FIG. 12 is a diagram of a heat transfer enhancement test apparatus300 in accordance with another embodiment of the present invention. Theheat transfer enhancement test apparatus 300 includes an FCS 302, whichis wrapped around the leading edges of heat exchanger fins 304 and 306.In this specific embodiment, the heat exchanger fins 304 and 306 are 250mm wide.

[0059] A 3-m/s approach velocity of ambient atmospheric air 308 througha 12.5-mm wide fin passage was used while the upper heat exchanger fin304 was heated or cooled. The heat transfer coefficients were deducedfrom direct measurement of fin surface heat flux and air temperatures.The passage pressure drop is between the ambient air and exit of thepassage. Application of the FCS 302 was seen to reduce the pressure dropby about 32% while increasing fin surface heat transfer coefficientsbetween 43% and 127%. The FCS 302 achieves this by destroying thesimilarity of temperature and velocity profiles (i.e., Reynolds analogy)through the sustenance of a thin vortex 310, through turbulence spectrummodification, near the fin surface. Heat flows easily across thisvortex, which also allows the main flow through the passage to proceedunabated as compared to the clean fin surface. The following illustratesheat transfer characteristics with and without the FCS 302.

[0060] Clean Fins CLEAN FINS (No FCS) FIN WITH FCS Pressure Drop along50-mm passage: (ΔP) = 16.0 ± 0.1 Pa (ΔP) = 10.9 ± 0.1 Pa Average HeatTransfer Coeft (Top Heated):   h = 38.7 ± 1.2 W/m2-K   h = 55.5 ± 1.7W/m2-K Average HeatTransfer Coeft (Top Cooled):   h = 18.5 ± 4.0 W/m2-K  h = 42.0 ± 5.5 W/m2-K

[0061]FIG. 13 is a top-view diagram of a multi-fin heat sink 350 inaccordance with another embodiment of the present invention. Themulti-fin heat sink 350 includes an FCS 352, which is wrapped aroundheat exchanger fins 354. The heat exchanger fins 354 are coupled to abase 356. In operation, heat transfer from the fins to a fluid, orvice-versa, is enhanced, while reducing the fin-passage pressure drop inthe fluid.

[0062]FIG. 14 is a side-view diagram of the multi-fin heat sink 350 ofFIG. 13. The FCS-enhanced fins 354 can be configured into a multi-finheat exchanger in a variety of ways. For example, the fins can bestaggered as shown in FIG. 13. The fins 354 can form a plurality of flowpassages. The flow passages can be parallel. The principal flow throughthe flow passages can also have a component parallel to the localgravitational field thereby creating a compact natural convectionsurface. In another embodiment, the FCS can be coupled to fins on a heatpipe, fins on a tube carrying a hot or cold heat transfer fluid, or tothe leading edge of one or more blades of a fan.

[0063] According to the system and method disclosed herein, the presentinvention provides numerous benefits. For example, it can enhance heattransfer in a variety of applications while minimizing or lowering thedrop in flow pressure, or reduce aircraft wing drag or make fans moreefficient and quiet.

[0064] Although the present invention has been described in accordancewith the embodiments shown, one of ordinary skill in the art willreadily recognize that there could be variations to the embodiments andthose variations would be within the spirit and scope of the presentinvention. For example, any of the embodiments shown could be used in avariety of applications and its use would be within the spirit and scopeof the present invention. Accordingly, many modifications may be made byone of ordinary skill in the art without departing from the spirit andscope of the appended claims.

[0065] References

[0066] 1. Carpenter, P. W., Lucey, A. D. and Davies, C., “Progress onthe Use of Compliant Walls for Laminar Flow Control,” J of Aircraft,Vol.38, No.3, 2001, pp. 504-512.

[0067] 2. Kays, W. M. and London, A. L. “COMPACT HEAT EXHANGERS- 3^(rd).Edition,” McGraw Hill, New York, 1984.

[0068] 3. Sinha, S. K., Wang, H., and Zou, J., “Interaction of an ActiveFlexible Wall with separating Boundary Layers,” AIAA Paper 99-3594,June-July 1999.

[0069] 4. Sinha, S. K., “Flow Separation Control with Microflexural WallVibrations,” Journal of Aircraft, Special Issue on Flow Control (Vol.38,No.3., May-June-2001) pp. 496-503.

[0070] 5. Sinha, S. K., and Zou, J., “On Controlling Flows withMicro-Vibratory Wall Motion,” AIAA paper AIAA-2000-4413, August 2000.

[0071] 6. Sinha, S. K., “System for Efficient Control of Separationusing a Driven Flexible Wall,” U.S. Pat. No. 5,961,080, awarded Oct. 5,1999

What is claimed is:
 1. A flexible composite sheet comprising: amembrane; a substrate coupled to the membrane; and a plurality of ridgescoupled between the membrane and the substrate, wherein the membrane isexposed to a region in a flow of fluid where the streamwise flowpressure gradient changes from favorable to adverse; wherein a vibratorymotion is induced from the flow to at least one segment of a membranespanning a distance s, wherein the vibratory motion is reflected from atleast one segment of the membrane to the flow, and; wherein a reductionin fluctuations is caused in the flow pressure gradient and freestreamvelocity U at all frequencies except around f, where f≈U/s.
 2. Theflexible composite sheet of claim 1 wherein the flexible composite sheetcan be wrapped around a leading edge of a plate facing an incoming flowof fluid.
 3. The flexible composite sheet of claim 1 wherein theflexible composite sheet can be wrapped around an aerodynamic surfacewhere a flow pressure gradient changes from favorable to adverse, inorder to reduce the intensity of flow-induced unsteady forces and reduceaerodynamic drag.
 4. The flexible composite sheet of claim 3 wherein theaerodynamic surface is a portion of a wing, in order to reduceaerodynamic drag, increase wing lift to drag ratio, delay the onset offlow separation and stall and reduce the intensity of flow-inducedunsteady forces on the wing.
 5. The flexible composite sheet of claim 1wherein the fluid can be a gas, vapor, mixtures of gases and vapors or aliquid.
 6. A heat sink comprising: a fin having a leading edge; and aflexible composite sheet coupled to the fin leading edge, wherein heatis transferred from the fin to a fluid without incurring an addedpressure drop in the fluid.
 7. The heat sink of claim 6 wherein theflexible composite sheet comprises: a membrane; a substrate coupled tothe membrane; and a plurality of ridges coupled between the membrane andthe substrate, wherein the membrane is exposed to a region in a flow offluid where the streamwise flow pressure gradient changes from favorableto adverse; wherein a vibratory motion is induced from the flow to atleast one segment of a membrane spanning a distances, wherein thevibratory motion is reflected from at least one segment of the membraneto the flow, and; wherein a reduction in fluctuations is caused in theflow pressure gradient and freestream velocity U at all frequenciesexcept around f, where f≈U/s.
 8. A heat exchanger comprising: aplurality of fins, wherein the fins are staggered; and a flexiblecomposite sheet coupled to some or all of the fins of the plurality offins, wherein heat is transferred from the fin to a fluid withoutincurring added flow pressure drop.
 9. The heat exchanger of claim 8wherein the plurality of fins form a plurality of flow passages.
 10. Theheat exchanger of claim 9 wherein the plurality of flow passages areparallel.
 11. The heat exchanger of claim 9 wherein a principal flowthrough the plurality of flow passages has a component parallel to thelocal gravitational field thereby creating a compact natural convectionsurface.
 12. The heat exchanger of claim 9 wherein the fins have acurved profile.
 13. The heat exchanger of claim 9 wherein the (FCS) isconfigured in a fish-scale pattern.
 14. A heat spreader comprising: aheat pipe; a plurality of fins coupled to the heat pipe; and a flexiblecomposite sheet coupled to at least one of the fins of the plurality offins, wherein heat is transferred from the fin to a fluid withoutincurring a pressure drop in the fluid.
 15. A fan comprising: aplurality of fan blades; and a flexible composite sheet coupled to atleast one leading edge of a fan blade of the plurality of fan blades,wherein heat is transferred from the fin to a fluid without incurring apressure drop in the fluid and wherein the fan can be made to be morequiet and efficient.
 16. A method for transferring heat, the methodcomprising the steps of: (a) providing a membrane coupled to a substrateand a plurality of ridges coupled between the membrane and thesubstrate; (b) inducing a vibratory motion from fluctuations in the flowvelocity U through the flow pressure gradient to at least one segment ofa membrane spanning a distance s; and (c) reflecting the vibratorymotion from at least one segment of the membrane to the flow pressuregradient to sustain pressure fluctuations in the flow pressure gradientat a frequency f, where f≈U/s, wherein heat transfer from a solidsurface downstream of the membrane segment to the fluid is enhancedwhile attenuating the flow pressure drop.
 17. The method of claim 16wherein the at least one segment of a membrane vibrates at a vibratingfrequency and the flow pressure gradient fluctuates at the substantiallythe same vibrating frequency.
 18. The method of claim 16 wherein aresulting flow velocity in the flow pressure gradient fluctuates arounda frequency f=U/s, wherein U is the freestream velocity above themembrane and s is the distance between adjacent ridges on a substrateover which the membrane resides.
 19. The method of claim 18 wherein fdoes not coincide with a fundamental flexural natural frequency of thesegment of the membrane.